Wireless radio communications systems enable many mobile stations or other remote subscriber stations to connect to networks, such as land-based wire-line telephone systems and/or digital information services, for information communication. For example, conventional wireless air-interfaces, such as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA), provide for controlled access to communication bandwidth by remote subscribers.
In wireless communications that are confined to a given space, throughput is generally considered to be bounded by Shannon's theorem, i.e., the maximum transmission rate in bits per second is given by Wlog2(1+SN), where W is the bandwidth and SN is the signal-to-noise ratio. For example, high data rate transmission (in given channel bandwidth) limits spreading and orthogonal coding such that the higher the data rate the less spreading is possible and, accordingly, less orthogonal codes are available. When Wideband CDMA (W-CDMA) or cdma2000 reach their highest data rate, very little spreading is used, and a small number of orthogonal channels are available. Similarly, General Packet Radio Services (GPRS) and Enhanced Data GSM Environment (EDGE) are good examples of high data rate transmission that is currently limited by the Shannon's theorem bounds.
The wireless communication industry, working with high order modulation and coding techniques, such as those implemented in the W-CDMA, GPRS, and EDGE air interfaces mentioned above, has gotten very close to the Shannon's theorem bounds. Accordingly, it is expected that significant increases in throughput are unlikely to be achieved by further manipulation of such techniques with respect to communications confined to a given space alone. However, increases in wireless capacity may be provided by segregating transmissions to different confined spaces (spatial segregation).
Due to spreading and orthogonal codes in CDMA systems (that gives the interference characteristics of random noise), it is possible to achieve some increase in capacity using spatial segregation associated with fixed sectorization, i.e., dividing the cell into multiple sectors to reduce interference level. However, spatial segregation, such as that associated with fixed sectorization, alone is insufficient to provide maximum increased capacity because of the unpredictable nature of wave propagation. Specifically, the scattering of radiated signals produces multipath conditions that can disrupt communications even in systems employing spatial segregation.
Spatial Division Multiple Access (SDMA) is a way to increase system throughput by segregating transmission space. Specifically, SDMA allows for multiple transmissions, separated by antenna beams (as opposed to frequency, time, or code), to be simultaneously supported. Examples of SDMA methods employing adaptive antenna arrays are described in U.S. Pat. Nos. 5,471,647 and 5,634,199 to Gerlach et al.; an article by M. C. Wells, entitled: “Increasing the capacity of GSM cellular radio using adaptive antennas”, IKE (UK) Proc. on Comm. Vol. 143, No. 5, October 1996, pp. 304-310; and an article by S. Anderson, B. Hagerman, H. Dam, U. Forssen, J. Karlsson, F. Kronestedt, S. Mazur and K Molinar, entitled: “Adaptive Antennas for GSM and TDMA Systems”, IEEE Personal Communications, June 1999, pp. 74-86, all of which are incorporated by reference.
Unfortunately, forward link beam forming as used in providing SDMA for mobile applications presents multiple challenges. For example, when the reverse link frequency differs from the forward link frequency, it may be difficult to accurately estimate the forward link direction and angle spread based on reverse link metrics. Signal fading in mobile applications causes the transmission channels to change rapidly, thereby limiting the efficiency of “feedback type” forward link beam forming estimation. This problem persists for Time Division Duplex (TDD) type systems. Accordingly, implementation of SDMA in wireless communication systems to provide improved capacity has been limited.
Moreover, it has been discovered in arriving at the present invention that, when beam shapes are relied upon to provide isolation among different transmissions, multipath can deteriorate the isolation and thereby introduce intra-cell interference. This intra-cell interference may disrupt the transmission, hence diminishing or eliminating the advantage of beam forming all together.
For example, in a typical multipath rich environment, the beam shapes used in providing SDMA cannot guarantee sufficient inter-beam interference rejection as the signals associated with one subscriber's station may be reflected from structures and penetrate areas that are assumed to be out of reach of that subscriber's beam. Furthermore, in some situations, non-line of sight scattering may actually enable the connection with a particular subscriber. In such a situation the beam-width associated with such a non-line of sight link may not be reduced as much as for line of sight conditions, resulting in increased multipath intra-cell interference.
A need therefore exists in the art for systems and methods which provide segregation of an area in which communications are provided to thereby provide increased capacity. A further need exists in the art for such systems and methods to segregate the space based on multipath conditions to thereby provide adaptive segregation.